Systems and methods for converting carbon dioxide into chemical feedstock

ABSTRACT

In one embodiment, carbon dioxide is converted into a chemical feedstock by providing a mixture of plasmonic material and oxygen-conducting material, exposing the mixture to sunlight so that solar energy is absorbed by the plasmonic material which then heats the oxygen-conducting material so that oxygen vacancies are generated, passing carbon dioxide through the mixture, and the oxygen-conducting material removing oxygen atoms from the carbon dioxide to form carbon monoxide.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority to U.S. Provisional Application Ser.No. 61/652,991, filed May 30, 2012, which is hereby incorporated byreference herein in its entirety.

BACKGROUND

Power plants, such as coal and natural gas plants, generate largeamounts of carbon dioxide. Because carbon dioxide is a greenhouse gas,it is desirable to limit the amount of carbon dioxide that is releasedinto the atmosphere. Although a seemingly simple solution to the problemof limiting carbon dioxide release would be to convert the carbondioxide into other compounds, which could be used in variousapplications, carbon dioxide is an extremely stable molecule and istherefore difficult to break down into other components. It cantherefore be appreciated that it would be desirable to have a system andmethod for converting carbon dioxide into other compounds.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure may be better understood with reference to thefollowing figures. Matching reference numerals designate correspondingparts throughout the figures, which are not necessarily drawn to scale.

FIG. 1 is a graph that illustrates the energy for conversion of carbondioxide into carbon monoxide and oxygen as a function of temperature.

FIG. 2 is a schematic view of an embodiment of system for convertingcarbon dioxide into chemical feedstock.

FIG. 3 is a flow diagram of an embodiment of a method for convertingcarbon dioxide into chemical feedstock.

FIG. 4 is a block diagram of a system for converting carbon dioxide intochemical feedstock and for producing a product using the feedstock.

FIG. 5 comprises graphs that illustrate conversion of carbon dioxide(from 10% CO₂/He) to carbon monoxide using partially-reduced Ag/orPt/(La_(0.75)Sr_(0.25))CoO_(3-δ).

FIGS. 6( a) and 6(b) are high-resolution transmission electronmicroscopy (HR-TEM) images of Ag—Cu nanoparticles with a sizedistribution of 14.8±5.4 nm (derived from a population of 100 particles)and a composition of Ag:Cu=1:1.

FIG. 7 is a graph of normalized extinction spectra for Ag—Cunanoparticles at different percentages of Cu. The solid lines show theextinction spectra of Ag—Cu nanoparticles in solution and the dottedline shows the extinction spectra of Ag—Cu nanoparticles with 33% Cu onAPS-coated glass slides.

FIGS. 8A and 8B comprise HR-TEM images of (A) approximately 3-nm Agnanoparticles and (B) approximately 3-nm Ag—Pt nanoparticles (200 nmscale bar in both images).

FIG. 9 is a graph of powder x-ray diffraction data collected from(La_(1-x)Sr_(x))CoO₃ samples. (The intensity signal values have beenoffset for viewing purposes.)

FIG. 10 is a graph of hydrogen temperature-programmed reduction (TPR) ofAg/and Pt/(La_(0.75)Sr_(0.25))CoO₃ samples. (The signal for consumptionvalues has been offset for viewing purposes.)

FIG. 11 is a graph of a temperature-programmed oxidation (TPO) profileof uncharred LaCoO₃.

FIG. 12 is a graph of x-ray diffraction patterns of fresh(La_(1-x)Sr_(x))CoO_(3-δ) powders and (La_(0.75)Sr_(0.25))CoO_(3-δ)after isothermal reduction at 500° C. and CO₂ conversion at 850° C.

FIG. 13 is a graph of TPR with 5% H₂/N₂ of (La_(1-x)Sr_(x))CoO₃ samples.

FIG. 14 is a graph of changes in a La_(0.75)Sr_(0.25)CoO₃ crystallinestructure while it is heated in 10% H₂/He.

FIG. 15 is a graph of carbon monoxide generation with 10% CO₂/He usingpartially-reduced (La_(1-x)Sr_(x))CoO_(3-δ).

DETAILED DESCRIPTION

As described above, it would be desirable to have a system and methodfor converting carbon dioxide into other compounds. Disclosed herein aresystems and methods for converting carbon dioxide into useful chemicalfeedstock, such as carbon monoxide, which can be used in industrialprocesses including fuel synthesis and the production of carbon fiberproducts. In some embodiments, carbon dioxide from a source, such as apower plant, is passed through catalyst material that removes oxygenatoms from the carbon dioxide molecules to form carbon monoxide. In someembodiments, the catalyst material is an intimate mixture ofoxygen-conducting material and plasmonic material that absorbs solarenergy. In such cases, the heat required for the reaction can beobtained from the solar energy.

In the following disclosure, various specific embodiments are described.It is to be understood that those embodiments are exampleimplementations of the disclosed inventions and that alternativeembodiments are possible. All such embodiments are intended to fallwithin the scope of this disclosure.

Sustainable chemical transformation of carbon dioxide is a challengefacing our society. Although carbon dioxide is abundant, it is a stablemolecule that requires a significant driving force to break down. Thesplitting of carbon dioxide into carbon monoxide and molecular oxygen isan unfavored reaction under typical conditions. This is illustrated inFIG. 1, which plots the Gibbs free energy for the conversion of carbondioxide into carbon monoxide and oxygen as a function of temperature. Inview of the need to break down carbon dioxide, a chemical process hasbeen developed for transforming carbon dioxide into carbon monoxide andoxygen. A catalyst material comprising an oxygen-conducting material isused to remove oxygen atoms from the carbon dioxide. In someembodiments, the catalyst material is a composite catalyst thatcomprises an intimate mixture of oxygen-conducting material andplasmonic material. In such cases, solar energy can be used to heat theplasmonic material, which in turn provides the heat required to produceoxygen vacancies in the oxygen-conducting material that can removeoxygen atoms from the carbon dioxide.

When solar energy is to be utilized in the carbon dioxide conversion,the catalyst material can be contained in a solar reactor through whichcarbon dioxide can flow and into which solar energy can pass. FIG. 2shows an example embodiment of a carbon dioxide conversion system 10that generally comprises a solar reactor 12 that contains the catalystmaterial. In some embodiments, the reactor 12 comprises a tubularcontainer having an inlet 14 and an outlet 16 and being made of atransparent or translucent material, such as quartz or a ceramicmaterial, so that solar energy can pass through the walls of the reactorand into the catalyst material. The reactor 12 can withstand hightemperatures, such as up to approximately 800° C.

The oxygen-conducting material has the capability of conducing oxygenand storing in oxygen vacancies. This capability increases withincreasing temperature and decreasing oxygen partial pressure. In someembodiments, the oxygen-conducting material comprises one or moreperovskite-type oxides and/or spinel-type oxides. The perovskite-typeoxide can have an ABO₃ molecular formula and the spinel-type oxide canhave an A₂BO₄ molecular formula, with the A site being occupied by alanthanide or alkaline earth metal, such as lanthanum, strontium,barium, cerium, or calcium, and the B site being occupied by atransition metal such as cobalt, iron, nickel, copper, manganese,vanadium, titanium, zinc, or chromium. Both the A and B sites can bepartially substituted by up to two metals. Therefore, theperovskite-type or spinel-type oxides can be easily customized toachieve desired properties, such as high oxygen mobility. An advantageof using perovskite-type or spinel-type oxides rather than a metal/metaloxide pair is that, for perovskite-type and spinel-type oxides, theoxygen release and storage are gradual processes with respect totemperature. Moreover, stability may be improved by working within asingle-phase as compared to a metal/metal oxide pair oxidation andreduction process.

In some embodiments, carbon dioxide conversion may be optimal when thesurface basicity of the oxide-phase is maximized in terms of its basicsite density and strength. Because carbon dioxide is a stable molecule,its activation is likely to be the rate-determining step.Perovskite-type and spinel-type oxides have many formulations yieldingsites of a basic nature. Moreover, surface basicity and oxygen mobilityhave been linked. Thus, generally speaking, the oxide material with thehighest surface basicity may also have the most oxygen vacancies and alower onset temperature of oxygen vacancy formation.

The plasmonic material absorbs visible light and therefore can be usedto heat and support the oxygen-conducting material. When exposed tovisible light near the peak plasmonic wavelength of the material, theplasmonic material can generate large amounts of heat that can dissipateto the oxygen-conducting material, causing its temperature to increaseand oxygen vacancies to form through the release of molecular oxygen. Insome embodiments, the plasmonic material comprises a noble metal such asgold, silver, platinum, or copper. Such noble metals exhibit uniqueoptical properties because of their ability to excite localized surfaceplasmons. In some embodiments, two or more noble metals can be alloyedtogether or one or more noble metals can be alloyed with another metal,such as copper, aluminum, chromium, or zinc. In some embodiments,smaller metal particles, such as metal nanoparticles having nominaldimensions (e.g., diameters) of approximately 3 to 10 nm, are usedbecause they are more likely to generate localized heat as opposed toinitiating a radiative Rayleigh scattering process.

The wavelength corresponding to the absorption peak and the scatteringefficiency of the metal particles are important factors that dictate theplasmonic properties of the material. These properties can bemanipulated by controlling the particle composition, size, aspect ratio,shape, particle-to-particle distance, and surrounding dielectric medium.In some embodiments, bimetallic nanoparticles are an attractive optionfor manipulating the plasmonic properties because they offer additionaldegrees of freedom for tuning their optical properties by alteringatomic composition and atomic arrangement. The ability to tune theposition of the absorption peak over a wide range of wavelengths enablesthe capture of the entire wavelength range of the solar spectrum and isuseful for solar applications. The use of composition rather than sizeand/or shape advances the potential for controlling the light absorptionwavelengths to temperatures not previously possible.

The temperature to which the plasmonic material rises when exposed tolight at its peak plasmonic wavelength depends upon the particularmaterial that is used. In some embodiments, however, the selectedmaterial can rise in temperature of approximately 5 to 250° C. inresponse to plasmonic absorption of visible light.

Various ratios of oxygen-conducting material and plasmonic material canbe used to form the catalyst material. In some embodiments, the catalystmaterial comprises approximately 80 to 95% oxygen-conducting materialand 5 to 20% plasmonic material by weight.

When the composite catalyst material cools, as when it is no longerexposed to sunlight, the temperature of the oxygen-conducting materialand the number of its oxygen vacancies decrease. Carbon dioxide isconverted to carbon monoxide because of re-oxidation of the material.Thus, carbon dioxide becomes enriched in either carbon monoxide oroxygen depending upon the light exposure and the resulting temperaturechange. With the process driven by visible light absorption, the energylimitations typically associated with the conversion of carbon dioxideare alleviated.

FIG. 3 illustrates an example method for converting carbon dioxide intocarbon monoxide that reflects the discussion of FIG. 2. Beginning withblock 20, an appropriate solar reactor is formed. As described above,the reactor can be a translucent or transparent container that is madeof a high-temperature material, such as quartz or a ceramic material.Once the reactor has been formed, it can be filled with a compositecatalyst material that comprises oxygen-conducting material and aplasmonic material, as indicated in block 22. Example oxygen-conductingmaterials and plasmonic materials have been identified above.

Next, with reference to block 24, the reactor is heated to raise thetemperature of the catalyst material to a first elevated temperature. Byway of example, the catalyst material can be electrically heated to atemperature of approximately 250 to 550° C. Once the first elevatedtemperature has been reached, the reactor can be exposed to solarradiation to further increase the temperature of the catalyst material,as indicated in block 26, to a second elevated temperature at whichoxygen vacancies will be formed in the oxygen-conducting material. Insome embodiments, the reactor can simply be placed in a location inwhich it is immersed in sunlight. In other embodiments, a solarconcentrator (not shown) can be used to focus the sun's rays on thereactor. As the sunlight passes through the walls of the reactor, theenergy of the sunlight is absorbed by the plasmonic material, whichexhibits plasmonic absorption of the visible light. In some embodiments,this absorption increases the temperature of the plasmonic material byapproximately 5 to 250° C. and its heat dissipates to theoxygen-conducting material with which the plasmonic material isintimately mixed. In some embodiments, the oxygen-conducting material isheated to a temperature of approximately 200 to 800° C.

At this point, oxygen within the reactor formed by the creation of theoxygen vacancies can be purged from the reactor, as indicated in block28. Next, the catalyst material is enabled to cool, as indicated inblock 30. In some embodiments, such cooling can be achieved by blockingthe solar radiation so that it does not impinge upon the reactor. Inother embodiments, the amount of heat supplied to the reactor can bereduced.

Referring next to block 32, carbon dioxide gas is passed through thereactor so that it flows through the catalyst material. Theoxygen-conducting material removes oxygen atoms from the carbon dioxideto convert it to carbon monoxide. Accordingly, carbon monoxide can beoutput from the reactor, as indicated in block 34. The above-describedcycle can then be repeated as desired to continue the carbon dioxideconversion process.

FIG. 4 depicts a further system 30 that illustrates an exampleapplication of the system 10 of FIG. 2 and the method of FIG. 3. Thesystem 30 shown in FIG. 4 includes an emissions source 32, such as apower plant, that outputs carbon dioxide. The carbon dioxide from thesource 32 can be purified by a carbon dioxide separation system 34 andthe purified carbon dioxide can be provided to the solar reactor 12 ofthe carbon dioxide conversion system 10. As indicated in FIG. 3, thereactor 12 is exposed to solar radiation provided by the sun and oxygencan be generated by the reactor once the temperature of theoxygen-conducting material has risen to an appropriate temperature, suchas approximately 200 to 800° C. and, as indicated in FIG. 4, can beprovided to another plant or stored for later use in other applications.When the reactor is permitted to cool, for example, when the reactor isshielded from the sun or after the sun sets, carbon dioxide can bepassed through the reactor and carbon monoxide can be output from thereactor.

As is further shown in FIG. 4, the carbon monoxide output from thereactor 12 can be provided along with steam to a water/gas shift (WGS)reactor 36 that operates at a temperature of approximately 200 to 500°C. and produces hydrogen from the steam. As indicated by the dashedline, the steam can alternatively or additionally be input into thesolar reactor 12 along with the carbon dioxide to generate hydrogen. Insuch a case, the WGS reactor 36 may not be necessary. Hydrogen andcarbon monoxide, for example in a 2:1 ratio, can be provided to aFischer-Tropsch synthesis (FTS) reactor 38 that operates atapproximately 220 to 350° C. to produce one or more liquid fuels.

At the temperatures described in relation to FIG. 4, there is littleconcern for the morphology change, including aggregation of theplasmonic phase, because plasmonic nano-particles are stable under theseconditions, even on more inert supports such as silica. In addition tothe solar heating via plasmonic phase, a catalytic effect caused by theaddition of the noble metals may occur. As shown in FIG. 5, it has beenverified that silver (Ag) and platinum (Pt) both cause a major decreasein the CO₂ conversion temperature. Isothermal CO₂ conversion studiesalso support these findings. With CO₂ conversion occurring attemperatures as low was 400° C. (for Pt, not optimized particlemorphology and bimetallic composition), it can be appreciated that thereis great potential for the process shown in FIG. 4. In alternativeembodiments, the solar reactor 12 can be conventionally heated to atemperature of approximately 250° C. and then the chemistry would beinduced through additional heating via exposing the plasmonic phase tovisible light.

Recently, ceria and its inherent non-stoichiometry at elevatedtemperatures has been explored for CO₂ conversion. An issue with ceriais that solar heating to generate oxygen vacancies from ceria occurs atextremely high temperatures (1,800 K) and requires concentrated solarlight. As demonstrated by the data of FIG. 5, however,metal/perovskite-type oxide composites can generate oxygen vacancies andconvert carbon dioxide at temperatures greater than 1,000 K lower thanceria. This finding indicates that concentrated solar energy is notnecessary and that natural light can be used.

The above-described carbon dioxide conversion approach is innovative onseveral levels. First, thermally-driven separation induced by visiblelight absorption is a novel concept. Second, the use of bimetallicparticles as a way to control the peak plasmonic absorption wavelengthis a better approach than the traditional methods, such as alteringparticle size and shape, because of the temperatures needed for theconversion. Third, the use of perovskite-type oxides for carbon dioxideseparation is largely unexplored. As compared to using the phase changeof a metal/metal oxide pair, temperature sensitivity and long-termstability are enhanced by using a change in oxygen content within asingle phase, as is possible with perovskite-type oxides. Thecombination of these innovations provides a materials platform thattransforms the current limitations in carbon dioxide conversion.

Preliminary studies were performed experimentally and computationally onthe synthesis, characterization, and properties of silver nanoparticlesand experimentally on the synthesis, characterization, and properties ofcomplex perovskite-type metal oxides. Ag—Cu nanoparticles were found toserve as excellent candidates for metal enhanced fluorescence (MEF)because of their interesting optical properties. Ag—Cu nanoparticles ofapproximately 15 nm in average size were synthesized by colloidal routes(see, e.g., FIG. 6). These particles were used to demonstrate a simpletechnique to tune the brightness of a luminophore by modifying thecomposition and atomic arrangement. Both the breadth and location of thepeak of the surface plasmon resonance (SPR) spectrum of the Ag—Cunanoparticles were tuned (FIG. 7). Ag—Cu nanoparticles greater than 10nm were predicted to maximize the plasmonic enhancement with an optimalparticle size of approximately 50 nm being predicted in previous studiesconducted by the inventors. While the radiative decay used in MEF islarger for particles of approximately 50 nm, what is useful for thiscatalysis application is non-radiative decay, which is convenientlylarger in smaller particles.

Because small particles are generally desired to maximize the conversionper mass of catalytic materials, Ag-based bimetallic particles that aresmaller than 10 nm were synthesized. In particular, approximately 10-nmAg—Pd bimetallic nanoparticles and approximately 3-nm Ag and Ag—Ptnanoparticles were synthesized (see FIG. 8 for the latter two types ofparticles).

In addition to the above-described work, perovskite-type oxides withvarious La:Sr ratios were synthesized using a modified-Pechini process.Extensive calcination studies were performed to optimize the temperatureneeded to remove the templating agents without causing aggregation.Optimal calcination temperatures near 700° C. were identified, which ledto specific surfaces of approximately 5 to 10 m²/g (measured by standardnitrogen isotherms at T=77K and BET isotherm analysis). These values areconsistent with expected findings. The success of the synthesis wasconfirmed by x-ray diffraction (XRD, FIG. 12) and temperature-programmedreduction (TPR, FIG. 10). In the XRD profiles, samples with Sr contentof X=0 and X=0.75 were found to be tetragonal while the powders withX=0.25 and X=0.5 were found to be cubic, which is recognized to favoroxygen mobility in the samples while maintaining phase stability. Thesample containing pure Sr showed an hexagonal crystalline profile.

Traces of other oxides: strontium carbonate and strontium oxalate wereobserved in samples with X=0.5 and X=0.75, demonstrating impure phasesas usually seen in these patterns. The TPR profiles are consistent withmultiple reduction steps. The low-temperature reduction (below ˜550° C.for all samples) represents a reduction of the tetravalent transitionmetal to the trivalent state. The high-temperature feature (600 to 800°C.) is associated with a combined reduction of the cobalt from atrivalent to divalent state and the loss of the perovskite phase thatcan include further reduction of cobalt to the metallic state. Thesematerials were used in initial studies, which showed that a La:Sr ratioof 3:1 was optimal compared to La:Sr ratios of 1:0, 1:1, 1:3, and 0:1(all ratios are molar) for low-temperature generation of oxygenvacancies and carbon dioxide conversion following oxygen vacancyformation.

The TPR profiles shown in FIG. 10 demonstrate that the deposited metalgreatly reduced the temperature at which oxygen vacancies form asindicated by the formation of water through the reaction of hydrogenwith lattice oxygen. Following reducing treatments, the oxygen vacanciesof the various materials were used to extract an oxygen atom from carbondioxide to form carbon monoxide. The additional oxygen vacancies and/orthe synergistic role of the deposited metal led to facile carbon dioxideconversion of the composite samples over the bare mixed oxides.

A further study was performed to test the capability of lanthanumcobaltites La_(1-X)Sr_(X)CoO₃ (0≦X≦1 in steps of 0.25) in reducingcarbon dioxide. As described below, the results show that carbon dioxidewas effectively converted to carbon monoxide while using pre-treatedoxides but the perovskite structure was lost due to the highpretreatment temperatures (600° C.). Different pre-treatmenttemperatures were examined (400, 500, and 600° C.) and it was determinedthat 500° C. is a viable temperature for the perovskite reduction.Furthermore, different carbon dioxide conversion temperatures weretested (650, 750, and 850° C.) and the highest carbon monoxideproduction was achieved when the reaction occurred at 750° C.

La(NO₃)₃ (Aldrich), SrCO₃ (Alfa Aesar), and CoCO₃ (Aldrich) weredissolved in an aqueous solution of citric acid (Aldrich) at 60° C. for2 hours. Ethylene glycol (Aldrich) was then added while stirring themixture at 90° C. for 12 hours following a Pechini synthesis withmodifications to the compounds, ratios (A site:B site:CA:EG=1:1:10:40).After a polymer was formed, the sample was heated in air at 450° C. for2 hours. Following this heating, a TPO-O₂ with 20% O₂/He was performedon the samples with an Sr content of X=0 and X=1 to determine theiroptimal charring temperature.

XRD was performed in a Phillips x-ray diffractometer with a CuKα(λ=0.154 nm) using a step size of 0.02 at 25° C. for both the freshsamples and after the isothermal CO₂ conversion. Multiple BET surfacearea studies were performed in a gas sorption system (Autosorb iQQuantachrome) for the fresh samples and after the isothermal CO₂conversion at 850° C. During the outgassing, the samples were heated ata ramp rate of 10° C./min until 200° C. and were held there for 3 hours.

TPR experiments were performed with a thermal conductivity detector in agas sorption system (Autosorb iQ Quantachrome). The analysis gas was amixture of 5% H₂/N₂ with a total 50 sccm flow. The temperature wasincreased at a ramp rate of 10° C./min from room temperature to 800° C.where it was held for 30 minutes.

Temperature-programmed oxidation with CO₂ (TPO-CO₂) experiments wereperformed in an MKS Cirrus mass spectrometer. After a pretreatment at600° C. under 10% H₂/He for 30 minutes, the sample was cooled down in Heto 100° C. Then 10% CO₂/He was flowed to the sample while it was heatedto 850° C. at a ramp rate of 10° C./min and was held at that temperaturefor 30 minutes. The total gas flow was held constant at 50 sccm.

After a pretreatment at 400° C., 500° C., or 600° C. under 10% H₂/He for30 minutes, the perovskites re-oxidation was studied in an MKS Cirrusmass spectrometer. The materials were ramp rated at 10° C./min in He tospecified temperatures (850, 750, and 650° C.). After stabilization atthe desired temperature, isothermal oxidation with CO₂ occurred. Thereaction environment was changed to 10% CO₂/He and was held for 30minutes.

In-situ XRD was performed on the La_(1-X)Sr_(X)CoO₃ (X=0.25) in a BrukerD8 x-ray diffractometer with a CuKα (λ=0.154 nm) while heating in aninert (50 sccm He) and reducing environment (5 sccm H₂/50 sccm He). Theheating ramp rate was approximately 25° C./min and the scan rate was2.5°/min. The heating ramp rate was 25° C./min and was held for 10minutes before the patterns were collected, at a scan rate of 2.5°C./min.

FIG. 11 shows a TPO performed on LaCoO₃ (X=0) after the esterificationreaction and the first heating at 450° C. At 360° C., oxygen consumptionthat forms a carbonate species can be seen, as is consistent withliterature. At this point, the sample was present as an amorphous powderas demonstrated by XRD (not shown). A small peak at 650° C. suggests theformation of more carbonate species, which might be due to traces of theprecursors still on the powder, which may be explained as acharacteristic of the crystallization yielding to theperovskite-structure.

A similar experiment was performed on the SrCoO₃ (X=1) sample and thecrystallization peak was seen at 700° C. From these experiments, 700° C.and 750° C. were chosen for the second heating temperature for thesamples with a Sr content of 0≦X≦0.75 and X=1 respectively.

The data shown in FIG. 12 shows the XRD profiles of the as-synthesizedmaterials and La_(0.75)Sr_(0.25)CoO_(3-δ) after isothermal CO₂conversion. Samples with a Sr content of X=0 and X=0.75 were found to betetragonal, while the powders with X=0.25 and X=0.5 were found to becubic, which favors them over the rest towards higher oxygen mobilitywhile maintaining phase stability. The sample containing pure Sr wasfound to be hexagonal. Traces of other oxides, including strontiumcarbonate and strontium oxalate, were observed in samples with X=0.5 andX=0.75, demonstrating impure phases as usually seen in these patterns.The surface areas of the samples were studied and no significant changewas observed between the fresh and post-reaction results. Fresh samplespresented an area ranging from 3 to 10 m²/g, while the post-reaction BETshowed between 3 to 15 m²/g.

The TPR profiles shown in FIG. 13 show the reducibility of the samplesas a function of temperature and X.

The peak positions and onset temperatures evidence the relationshipbetween the samples' reducibility and X. The highest onset temperatureis needed for the powder with X=0 while the lowest onset reductiontemperature is presented by the powders with X=0.25 and X=0.5. Likewise,onset for the second peak shows the same trend for each powder.

It is apparent that partial Sr substitution augments the reducibility ofthe powders by the instability created by divalent cations on the Asite, which leads to a coexistence in the B-site cation betweentetravalent and trivalent cobalt, which further reduces to divalent andmetallic cobalt, inducing phase changes. However, after an optimumamount of Sr has been incorporated (which increases the amount of alphaoxygen that can be desorbed, as for samples with X=0.25 and X=0.5),further Sr substitution decreases the amount of oxygen in the alpharegion. As seen in FIG. 13, samples with X=0.75 and X=1 have a lowerarea for the alpha oxygen and a higher area for beta oxygen comparedwith the rest of the samples. In each sample, the first peak isattributed to hydrogen consumption by the surface and lattice oxygen(which leads to the reduction of Co⁴⁺ to Co³⁺ and Co³⁺ to Co²⁺), whilethe second peak is attributed to the consumption of hydrogen from thebulk oxygen (reduction of Co³⁺ to Co²⁺ and Co⁰), which leads to thephase changes. Table 1 summarizes the temperature range in which bothalpha and beta oxygen peaks are seen in each sample.

TABLE 1 Summary of oxygen desorption peaks from TPR data. Sample Alphaoxygen Beta Oxygen (Sr content) (temp range ° C.) (temp range ° C.) X =0   350-550 550-800 X = 0.25 100-475 475-800 X = 0.5  100-475 475-800 X= 0.75 150-350 350-800 X = 1   150-350 350-800

In-situ XRD patterns of La_(0.75)Sr_(0.25)CoO₃ under hydrogen heatingshown in FIG. 14 demonstrate the evolution of the crystalline structurewhile undergoing hydrogen reduction. At 25° C., the powder maintains itscubic perovskite structure, which is still noticeable at 400° C. (underthe α-oxygen region) when the powder transitions to a perovskite-likephase SrLaCoO₄. At 500° C., (transition between α-oxygen region andβ-oxygen region) the perovskite phase is seen as traces (thecharacteristic peaks have shifted slightly to the left by consequence ofcell thermal expansion). The spinel-type SrLaCoO₄ phase has also beenreduced and the appearance of metallic cobalt and lanthanum oxide arenoticeable. Strontium might be present in an amorphous phase or in verysmall metallic particles, not detected by XRD. Further reduction above600° C. (β-oxygen region) shows the predominance of La₂O₃ and the totaldestruction of the initial perovskite and the spinel-type phase.

Because it takes longer to perform the in-situ XRD experiments, a purecubic perovskite phase might still be present at higher temperaturesduring the conventional TPR. The set of peaks at 50°, 51° and 55° seenon the profile are due to a quartz sample holder and remain constantthroughout the heating.

The TPO-CO₂ profiles of FIG. 15 show that carbon dioxide was effectivelyreduced to carbon monoxide on the H₂-pretreated oxides with the highestconversion peak being achieved at approximately 850° C.

As X increases, the peaks demonstrating CO₂ conversion become broaderand decrease in intensity, demonstrating that increasing the Srsubstitution increases the difficulty of the re-oxidation of theperovskite. A similar experiment with oxygen (not shown) revealed that,as the Sr content increases in the perovskite, so does the temperatureneeded for its re-oxidation with oxygen. SrCoO_(3-δ) shows two carbonmonoxide generation regions suggesting that the phase transitionsuffered by the oxide structure during the H₂-pretreatment does notfavor the conversion of CO₂ and the sample might be converted to otherphases instead of re-oxidizing with the carbon dioxide. The highstability of the X=0 sample shifts the re-oxidation of the perovskitetowards higher temperatures. Even though the peaks become broader withX, the lowest onset temperature approximately 560° C. is achieved by thethree samples with both cations on the A site.

A pure crystalline phase is preferred because it facilitates the studyover the phases actively involved in the reaction. For that reason, thesample with X=0.25 was chosen to study isothermal CO₂ to CO conversionat different temperatures.

A reduction/CO₂ re-oxidation matrix was created to explore the optimumconditions for the La_(0.75)Sr_(0.25)CoO₃ perovskite's isothermalreduction (400, 500, and 600° C.) and subsequent isothermal CO₂ reactiontemperatures (650, 750, and 850° C.). The formation of CO by moles ofoxygen available in the perovskite is shown in Table 2. The data isnormalized by the first 10 minutes of reaction because both the oxygendesorption process and CO₂ conversion are time sensitive.

TABLE 2 μ moles of CO generated per moles of oxygen in the perovskite.$\frac{\mu\;\text{moles of Co}}{\text{moles of}\mspace{14mu} O\mspace{14mu}{in}\mspace{11mu}{the}\mspace{11mu}{perovskite}}$Reduction Oxidation temperature Sample Temperature 650° C. 750° C. 850°C. X = 0.25 400° C. <5 24 39 500° C. <5 31 121  600° C. <5 20 45 X = 0.5500° C. — — 33 X = 0 500° C. — — 21

The results in Table 2 show that the highest CO₂ conversion is achievedat 850° C. when the perovskite is reduced for 30 minutes at 500° C. Forcomparative purposes, the isothermal CO₂ conversion was also performedwith these conditions on the samples X=0.5 and X=0 which presentedsimilar profiles for their re-oxidation with CO2. Is noticeable that theCO generation achieved with X=0.25 is higher than the achieved by X=0and X=0.5 demonstrating that the phases formed during theH₂-pretreatment of X=0.25 are favored towards the re-oxidation withcarbon dioxide.

A possible mechanism for the reduction of CO₂ could be explained byconsidering dissociative chemisorption energies of carbon dioxide onmetal surfaces. It has been suggested that the dissociation of carbondioxide in metallic cobalt is thermodynamically favored. This wouldexplain why the conversion reaches a maximum when the H₂-pretreatmentforms metallic cobalt (500° C. and 600° C.). Also, carbon dioxide iseasily physisorbed into perovskite structures due to its basicity, sinceat 500° C. there are still complex oxides present (perovskite andperovskite-like oxides), the carbon dioxide molecules could bephysisorbed into the structure of the La_(0.75)Sr_(0.25)CoO_(3-δ) andSrLaCoO₄ and then dissociated on the cobalt.

Post reaction XRD show the profiles of La_((1-X))Sr_(X)CoO₃ to identifythe phase transition after the isothermal conversion at 850 andH₂-pretreatments at 600° C. The original single-phase cubic perovskitestructure was not completely recovered during isothermal conversion at850 and the predominant phase obtained is LaSrCoO₄ which suggests thataggregation in the metallic cobalt could increase the particles sizewhich would alter the re-oxidation phase to a perovskite-like structurewith larger lattice parameters than the original cubic perovskite. Thepredominant phase (LaSrCoO₄) is formed from the reduction andre-oxidation reaction for the samples containing Sr 0.25≦X≦0.75. Thesamples containing Sr present SrCO₃, which increases with X. For X=1,only SrCO₃ is present in the structure.

The invention claimed is:
 1. A method for converting carbon dioxide intoa chemical feedstock, the method comprising: providing a mixture ofplasmonic material and oxygen-conducting material, wherein theoxygen-conducting material does not contain ceria and is a perovskitewith a general formula of ABO₃ or a spinel-type oxide with a generalformula of A₂BO₄ and wherein the A site element is selected from thegroup consisting of: beryllium, magnesium, calcium, strontium, barium,radium, lanthanum, praseodymium, neodymium, promethium, samarium,europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium,ytterbium, lutetium, and two-element combinations thereof; exposing themixture to sunlight so that solar energy is absorbed by the plasmonicmaterial which then heats the oxygen-conducting material so that oxygenvacancies are generated; passing carbon dioxide through the mixture suchthat the oxygen-conducting material thermally separates the oxygen atomsfrom the carbon dioxide to form carbon monoxide.
 2. The method of claim1, wherein the plasmonic material comprises a metal.
 3. The method ofclaim 2, wherein the metal comprises a noble metal.
 4. The method ofclaim 3, wherein the noble metal is gold, silver, copper, or platinum.5. The method of claim 2, wherein the metal comprises a bimetalliccombination of a noble metal and another metal.
 6. The method of claim1, wherein the B site is occupied by a transition metal.
 7. The methodof claim 6, wherein the transition metal is one of cobalt, iron, nickel,copper, manganese, vanadium, titanium, zinc, or chromium.
 8. The methodof claim 6, wherein the B site contains two transition metals.